[pdurrant]

Three logicians walk into a bar.
The publican asks "Do you three all want a beer?"

The first logician says "I don't know."
The second logician says "I don't know."
The third logician says "Yes."


(I think this is a joke. But perhaps only for logicians. For those who think it's more like a logic puzzle, the explanation is in spoilers:

Spoiler:

The first one knows he wants a beer, but doesn't know about the other two, so says "I don't know".
The second also wants a beer, and knows that the first wants a beer (as otherwise the first one would have said "No"), but doesn't know about the third.
The third one also wants a beer, and now knows that the other two also want beers, so says "Yes."

)